The adaptation of man and other animals to a planet with a 24 hour period of rotation has resulted in the evolution of a circadian timekeeping system that organizes the physiology and behavior of organisms. Individuals exposed to altered gravitational or temporal environments often exhibit physiological or behavioral deficits. Metabolic and psychological changes resulting from desynchronized circadian rhythms may significantly affect the health and well being of astronauts and cosmonauts. This project continues our investigation of the biomedical effects of alterations in gravity and lighting on metabolism and circadian rhythms of body temperature, heart rate, activity, feeding, drinking, and performance of the rhesus monkey. These data may suggest interventions that would alleviate adverse human responses to conditions such as spaceflight, jet-lag or shift work. We recently measured the metabolic energy expenditure of three monkeys flown in space for 14 days using the double labeled water method. The results from these animals indicated a 40% reduction of energy expenditure during spaceflight as compared to levels on Earth. We are currently measuring performance in the rhesus using the Psychomotor Test System (PTS) designed in the Language Research Center at Georgia State University. PTS is a computer generated series of sixteen tasks which test different aspects of performance (such as eye-hand coordination or short term memory). The tasks build upon previously acquired skills, and are learned sequentially. Four to eight months are required to fully train an animal to use this system. We have trained eight male rhesus in the use of PTS. In addition, we have collected baseline data on level and rhythms of performance in this species. We will use this data for comparison with performance exhibited in altered lighting and gravitational environments. *KEY*Biological rhythms, Temperature regulation, Gravity, Acceleration, Performance